Among novel therapies being explored for malignant glioma, oncolytic viruses possess relative tumor[unreadable] selectivity and are being tested in clinical trials. In prior years of funding, we have constructed herpes[unreadable] simplex viruses genetically engineered to replicate in tumor cells. Published phase I clinical trials for brain[unreadable] tumors have reported the safety of herpes viral vectors but offered little data concerning efficacy in humans.[unreadable] We have found that herpes viral replication can be enhanced by the drug, cyclophosphamide. In published[unreadable] and pilot experiments, the effect of cyclophosphamide (CPA) is rapid and does not require the presence of[unreadable] pre-existing immunity to the oncolytic virus. CPA leads to increased viral liters in multiple types of glial[unreadable] tumors (human, mice or rat) and results in increased survival. The effect appears to also occur with an[unreadable] oncolytic adenovirus. We hypothesize that normal host responses impede the process of oncolytic viral[unreadable] infection. In pilot data, macrophages appear to be a major determinant of such host responses. We have[unreadable] also dentified molecular markers (including IFNy, iNOS, VEGFR2, APN and versican) of the host (and[unreadable] possibly macrophage-mediated) response to oncolytic viral infection and have preliminary data that CPA[unreadable] acts as a sensitizer of viral oncolysis by limiting such responses. The CPA effect is relatively pleiotropic and[unreadable] there is a need to identify specific pathways that underlie the host processes that impede viral oncolysis and[unreadable] pharmacologic means to limit such responses. We thus propose the following aims: AIM 1-Validate changes[unreadable] in the molecular markers that are associated with viral oncolysis; AIM2- Determine the cellular infiltrates in[unreadable] tumor that are associated with viral oncolysis; AIM 3-Determine if CD68+ cells (macrophages/microglia)[unreadable] and their gene products limit viral oncolysis: AIM 4-Determine if CD56+ (NK cells) and their gene[unreadable] product, IFNY, limit viral oncolysis. We will employ quantitative RT-PCR, FACS, molecular imaging[unreadable] (Project 3, immunohistochemistry (Pathology Core) and genetic and pharmacologic analyses to prove or[unreadable] refute the stated hypotheses, using GLP- and GMP-grade oncolyic HSV (Vector Core B). If successful, this[unreadable] project will begin to elucidate at a molecular level how the host responds to a viral infection of its brain tumor[unreadable] and how this can be modulated to increase anticancer effects. This should be of significance for current and[unreadable] future clinical trials of cancer that employ this modality.[unreadable]